Thousands of people will sleep out later in a bid to raise awareness and funds to tackle homelessness.
The World’s Big Sleep Out event, spanning 52 countries, will see more than 50,000 people sleeping out all over the world.
Organiser Josh Littlejohn said the event would be the biggest achievement in his lifetime.
Mr Littlejohn started the Sleep Out event in Edinburgh four years ago with 270 people in Charlotte Square.
After three Big Sleep Outs in Scotland, this year the event has gone global – from Times Square in New York to the central park in New Delhi.
The aim is to raise millions of pounds for homeless and refugee causes and to increase pressure on governments to take action to help an estimated 100 million people globally who do not have a home.
Earlier this week Mr Littlejohn said it would be the last time he organised the event, saying he wanted to end on a high.
Mr Littlejohn, who also founded social enterprise Social Bite, said he had wanted to create a “Live Aid moment” with the sleep outs and that it had never been the plan to continue them long term.
He said this weekend’s World’s Big Sleep Out would fund 100 charities.
The money from previous years has been used to fund 831 flats across Scotland for homeless people as well as the Social Bite Village in north Edinburgh.
Hollywood actor Brian Cox is due to open the Edinburgh sleep out later at West Princes Street Gardens.
Will Smith and Dame Helen Mirren will lead the way by telling a bedtime story in Times Square in New York and Trafalgar Square in London respectively.
Mayor of London Sadiq Khan, presenter and model Lisa Snowdon, six-time Olympic champion Sir Chris Hoy, ITV News Presenter Julie Etchingham, singer Jamelia, author Giovanna Fletcher, Senior fire officer Dr Sabrina Cohen-Hatton and TV and radio presenters Kate Garraway, Cherry Healey, Angela Scanlon and Anita Rani are all due to take part.
Mr Littlejohn, said: “I am blown away by the response to The World’s Big Sleep Out and incredibly excited to have 50 cities all over the world taking part.
“It doesn’t matter if you are taking part in Times Square or in your back garden with your family, by sleeping out for one night we can simultaneously express our compassion for homeless people who have no other choice and raise lifesaving funds to make a difference.
“We can also send a message to the world’s political leaders that urgent action is required to address the human suffering that we each witness on our streets every day”.
Dame Helen Mirren said: “Homelessness on our streets is a real problem in the UK and for many nations throughout the world.
“The World’s Big Sleep Out campaign will play an important role in shining the political spotlight on the issues of homelessness and displacement whilst raising considerable funds to make a big difference.
“I’d like to encourage anyone who cares about this issue to get involved by sleeping out, in solidarity with those who have no choice.”